A. Spencer typed the wrong URL into Internet Explorer, and pop-ups came faster than he could close them.

You hit a rogue site, and I suspect that getting to it via a typo wasn't an accident. The crooks who run these sites will occasionally get domain names very similar to common Web destinations just to catch people making such common mistakes.

The big question is: Did the site do something worse than bombard you with advertising? If the problem persists, even after you've closed down your browser and rebooted your computer, then it most certainly left something malicious on your computer.

On the other hand, if rebooting appears to have fixed the problem, you still can't be sure that it didn't leave something malicious on your computer. See Remove a Virus or Other Malicious Infection for suggestions of where to go from here.